Some disk controllers and SCSI host adapters extend the capabilities of the system BIOS with their own BIOS. This allows the system to boot from an ESDI disk or a SCSI disk on the SCSI bus controlled by a SCSI host adapter.
A limitation of the system BIOS is that it will not boot an operating system that lies on or past the 1024th cylinder on the root hard disk. See ``BIOS support for disks larger than 1024 cylinders'' for more details.
The disk device drivers of some operating systems (but not SCO OpenServer) use the BIOS to access the hard disk in regular use. This means that they cannot access disk blocks that lie past the 1024th cylinder. Disk controllers which support logical block addressing or LBA (IDE drives with more than 1024 cylinders, most EIDE and UDMA drives, and all SCSI drives) can remap or translate the geometry of the disk so that it appears to the system to have less than 1024 cylinders.
On some machines, the disk geometry is stored in CMOS RAM (parameter RAM) on the motherboard. See ``Defining IDE and ESDI disk geometry in the BIOS'' for information on what you should do if the disk geometry information becomes lost from CMOS RAM. If a BIOS extension is used, the disk geometry may be stored on the disk controller or SCSI host adapter.
When booting from the hard disk, the BIOS reads the masterboot block to find out which active partition it should boot the system from. If the partition contains the SCO OpenServer system, the hdboot0, hdboot1, and boot bootstrap programs execute in sequence. If necessary, these programs can translate between different geometries defined for the root disk in the BIOS and the masterboot block. The boot program also passes on the geometry information obtained from the BIOS for use by disk device drivers.
If you move a root disk between machines or change its host adapter (if SCSI), the geometry defined in the BIOS may no longer match the geometry that the operating system previously used to access the disk. This may happen if you move a SCSI disk to a new host adapter that assumes a different disk geometry, or if you move an IDE drive to a different computer. In such cases, you may be able to boot the machine but be subsequently unable to use the disk because the information about the disk geometry stored in the masterboot block does not match that defined in the BIOS. You can write the BIOS disk geometry information to the masterboot block on the root hard disk as described in ``Writing a new masterboot block''.
If you do not want to change the geometry information stored in the masterboot block because you need to define the disk as having less than 1024 cylinders, you can override the root disk geometry information stored in the BIOS when you boot the system as described in ``Overriding the root disk geometry stored in the BIOS''.